1. Field of the Invention
The invention relates to a method for manufacturing a spin valve magnetoresistive head, and such head and a composite magnetic head manufactured by the method.
2. Description of Related Arts
In 1996, data recording density on a hard magnetic disk (HDD) exceeded 1 G bits/inch.sup.2. The success for such a high density recording was achieved by the development of a magnetoresistive (MR) head which enabled a higher read output power than then conventional thin film heads. The data recording density of a HDD has been still increasing with a surprising speed of 60% per year. In order to support this growth rate, further development of a magnetic head having a superior sensitivity is inevitable.
Under such circumstances, giant magnetoresistive films (hereinafter referred to as GMR film) have been noted as a promising candidate for the magnetic head because they can provide a large read signal. Of the various giant magnetoresistive films, a spin valve magnetoresistive film is the most promising since it is simple in structure and hence relatively easy to manufacture, and yet it may exhibit a large electric resistance change under a weak externally applied magnetic field compared to ordinary MR elements.
An MR head uses an MR film as a read head element, whose electric resistance may change under the influence of an externally applied magnetic field. Information recorded on the HDD may be detected by detecting the change in electric voltage caused by the electric resistance change of the MR head due to the external magnetic field from a HDD indicative of the information. A GMR head utilize GMR film instead of MR film.
A typical magnetoresistive head utilizes a spin valve magnetoresistive film (hereinafter referred to as spin valve GMR head), as disclosed for example in U.S. Pat. No. 5,206,590, Japanese Patent Publication Laid Open No. 6-60,336, and French Patent No. 95-5,699.
FIG. 1 illustrates such a prior art spin valve GMR head as mentioned above, showing in perspective view a major portion of the head. FIG. 2 shows a vertical cross section of the spin valve GMR head shown in FIG. 1.
The spin valve GMR head has a GMR film which is formed, in the order shown in FIG. 1, by the deposition of:
a under layer 11 of tantalum (Ta); PA1 a dual free magnetic layer 12 which consists of a first free magnetic layer 12a of a nickel-iron alloy (NiFe) and a second free magnetic layer 12b of a cobalt-iron-boron alloy (CoFeB); PA1 a non-magnetic metallic layer 13 of copper (Cu), PA1 a fixed magnetic layer 14 of a cobalt-iron-boron alloy (CoFeB), PA1 an anti-ferromagnetic layer 15 made of regular metals such as palladium-platinum-manganese (PdPtMn), and PA1 a cap layer 16 made of tantalum (Ta). PA1 forming a film including at least a free magnetic layer, a non-magnetic metallic layer, a fixed magnetic layer, and a magnetic domain control layer; PA1 subjecting the film to a first heat treatment under a magnetic field (referred to as first in-field heat treatment) to enhance magnetic anisotropy of the free magnetic layer; and PA1 subjecting the film to a second heat treatment under a magnetic field (referred to as second in-field heat treatment) and at a higher temperature than the maximum temperature used in the processes that precede the second heat treatment to fix the magnetization in the fixed magnetic layer. PA1 forming a film including at least a free magnetic layer, a non-magnetic metallic layer, a fixed magnetic layer, and a magnetic domain control layer; PA1 subjecting the film to a first heat treatment under a magnetic field (referred to as first in-field heat treatment) to fix the magnetization in the fixed magnetic layer; PA1 subjecting the film to a second heat treatment under a magnetic field (referred to a second in-field heat treatment) to enhance magnetic anisotropy of the free magnetic layer; and PA1 subjecting the film to a third heat treatment in the absence of an externally applied magnetic field (referred to as field-free heat treatment).
After these layers 11 through 16 are formed, the GMR film is subjected to a process for patterning the film into a generally planar configuration, and is provided with a pair of gold electrodes (Au) 17a and 17b on the opposite corners of the top most cap layer 16, resulting in a spin valve GMR head. In such spin valve GMR head, a region between the two electrodes 17a and 17b serves as a signal detection region (or sense region) S. In the specification of the present application, in order to specify the direction of an externally applied magnetic field applied to the GMR head, a coordinate system is defined such that Z axis is taken along the thickness of the GMR film, Y axis along the line passing through the two electrodes, and X axis in the direction perpendicular to the Y-Z plane, as shown in FIG. 1.
During the operation of the spin valve GMR head, a sense current Is is passed through the sense region S from the electrodes 17a to the electrode 17b. With this current, if the spin valve GMR head is in motion, near a magnetic recording medium (not shown) such as a magnetic disk for example, relative to the magnetic medium, the electric resistance of the spin valve GMR head is changed in response to the X component of the magnetic field Hsig indicative of a signal from the magnetic medium, resulting in a varying voltage across the spin valve GMR head which is the product of the varying resistance and the sense current. Thus, the signal magnetic field is detected in the form of the voltage change. In order to make the magnetic response of the spin valve GMR film (i.e. magnetoresistance of the spin valve GMR film) linear with the signal magnetic field Hsig, magnetization Mp of the fixed magnetic layer 14 is generally fixed in the direction of X axis by means of an exchange coupling with the anti-ferromagnetic layer 15. In the absence of signal magnetic field Hsig, magnetization Mf of the free magnetic layer 12 is directed along Y axis which is perpendicular to the magnetization in the fixed magnetic layer 14. The direction of the magnetization in the free magnetic layer 12 with zero signal magnetic field, which is now oriented in the direction of Y axis, is called direction of easy magnetization. The magnetization Mf of the free magnetization layer 12 is adapted to rotate in response to an externally applied field such as a signal magnetic field Hsig from the magnetic recording medium such that the resultant magnetoresistance change of the GMR film is linear with the signal magnetic field Hsig.
However, in the case where a regular anti-ferromagnetic material is used for the anti-ferromagnetic layer 15, as is the case for conventional spin valve GMR heads, it is necessary to heat treat the GMR film after the GMR film is formed to thereby fix the magnetization Mp of the fixed magnetization layer 14 in the direction of X axis, because regular metals may undergo a phase transition only at a relatively high temperature, changing their lattice structures from face centered cubic (FCC) lattice structure to favored Face Center tetragonal (FCT) structure for the fixation of the magnetization Mp. Such heat treatment is carried out under a magnetic field of about 2,500 Oersteds (Oe) after at least the layers 1 through 6 (from the under layer 1 to the cap layer 6) of the GMR film are formed.
In the next step, the GMR film is heat treated under the influence of an appropriate magnetic field directed in Y direction to enhance anisotropy of the magnetization Mf of the free magnetic layer 12.
Unfortunately, the heat treatment for enhancing the anisotropy of the magnetization Mf in the free magnetic layer 12 affects the magnetization Mp already set up in the fixed magnetic layer 14 in X direction, thereby disadvantageously disorienting the magnetization Mp, as shown in FIG. 3, away from X axis towards Y axis. For an ideal GMR film, the magnetization in the fixed magnetic layer 14 be fixed in the direction of X axis while the magnetization in the free magnetic layer 12 is directed along Y axis so that the two magnetizations are perpendicular to each other in the absence of any external signal magnetic field Hsig. Then the output of the spin valve GMR head would be a linear function of the input signal (or external signal magnetic field Hsig). However, if the magnetization in the fixed magnetic layer 14 is disorientated away from X axis towards Y axis from the beginning, the spin valve GMR head cannot provide a linear response to the externally applied magnetic field Hsig, and yields a distorted waveform of the output voltage.
As discussed above, the fixed magnetic layer 14 and the free magnetic layer 12 ideally have magnetizations perpendicular to each other in the spin valve GMR head in the absence of any externally applied magnetic field Hsig. In actuality, the angle between the two magnetizations of the fixed magnetic layer 14 and free magnetic layer 12 must be at least 70 degrees for any usable spin valve GMR head, as verified by examinations of many conventional spin valve GMR heads.